History of India China Border Disputes

There are various disputed areas along the Indian-Chinese border in the Himalyas. The one that's caused recent issues is Galwan Valley.
India, China border personnel meet at Ladakh - Jammu Links News

There are various disputed areas along the Indian-Chinese border in the Himalyas. The one that's caused recent issues is Galwan Valley. Normally neither side brings weapons into the area, but in late April tensions escalated when China set up camps and brought over artillery and vehicles in disputed territory. There were demands from the Indians that the Chinese leave. This lead to the sides verbally shouting at each other and even throwing rocks. The tensions have been growing over the past few weeks, with brawls and even detentions taking place. The escalation reached boiling point today however, when 20 Indian soldiers were killed in a clash. 

Sino-Indian border History

Historically, in that region on the Sino-Indian border, the border exactly was wasn't formally agreed and was a bit fuzzy, from what I can tell more from the countries themselves not really being sure and there being no absolute international standards, though in the 1860s a survey of the region, establishing what was called the Johnson Line, put Aksai Chin - just east of Ladakh - inside India. Due to shifting geopolitical conditions, including not wanting a border with Russia as it more properly entrenched itself in its eastern territory, British invited the Chinese Empire to take control of the no-man's land in the region between the Karakoram and Kunlun ranges. By 1892, the British had decided that they put their border at the Indus watershed, the water-parting from which waters flow into the Indus river system on one side and into the Tarim basin on the other. This was agreed in 1899 and Aksai Chin was ceded to the Chinese Empire. However following the power shifts in China after the emperor was overthrown in the Xinhai Revolution in 1911, and after WWI, the British reverted to using the Johnson Line (which included Aksai Chin as part of India) but made no efforts to assert control on the ground. In the following years, they used as many as 11 different boundary lines, as the political situation dictated. Then, following Indian independence in 1947, India used the Johnson Line as its official boundary, and in 1954 formally claimed Askai Chin, saying it had been part of the Ladakh region for centuries. This eventually led to the Sino-Indian War in 1962, which China won, solidifying its control over Askai Chin. This explains the enmity on the Indian side.

Tibet had been a (largely autonomous) part of the Chinese Empire since the 1720s, but when the emperor was overthrown in the Xinhai Revolution in 1911, Tibet declared their independence shortly after. Then in 1914 Tibet negotiated its border with the then British Raj, along the McMahon line (proposed by the British colonial administrator Sir Henry McMahon), ceding the region of Arunachal Pradesh (north-east of modern-day Bangladesh) to the Raj. Fast forward to the Chinese Civil War, and the Republic of China formed after the Xinhai Revolution was overthrown by the Chinese Communists, and the PRC as we know it is established. Viewing itself as it does as the rightful Chinese government, it therefore lays claim to all the territory that was part of the Chinese Empire, including Tibet. Therefore in 1950 the PRC invaded Tibet and it was formally integrated in 1951. Remember that the PRC claims all the land that was formerly part of the Chinese Empire. Therefore, it rejects the McMahon line negotiated by the independent Tibet, and claims Arunachal Pradesh. This was also part of the tensions that led to the Sino-Indian war in 1962, and explains the enmity on the Chinese side.

Therefore, India claims Askai Chin from China, and China claims Arunachal Pradesh from India. And so, in the PRC's expansionist fashion, they have been pushing the borders to regain Arunchal Pradesh and secure the region around Askai Chin. Recently, they moved various troops and arms into the disputed region, complaining of Indian infrastructure projects (since they claim it's theirs, so what is India doing building there), triggering diplomatic conflict with India, which has now escalated into small-scale conflicts between Indian and Chinese troops, with deaths of 20 Indian soldiers, including 1 officer, and at least 43 Chinese casualties.