As this article explains, Doklam sit at the intersection of India, China, and Bhutan.
Starting in June, a tiny piece of strategically important and until-now obscure Himalayan territory sitting at the intersection of India, China, and Bhutan became the site of the one of the most serious border standoffs between New Delhi and Beijing in three decades. As of July 12, 2017, the standoff continues, with no end in sight. Scores — potentially hundreds — of Indian Army and Chinese People’s Liberation Army troops remain at an impasse near the Doka La pass in Doklam. Nearly one month after the standoff began, details about the geography of the area and the motivations of all three governments involved remain murky.
Basically, the disputed started when the Chinese People’s Liberation Army constructed a road near the Indian border on a piece of territory that is disputed between China and Bhutan, so India sent troops to prevent the PLA from proceeding. This sparked the Indian Army to respond.
Though I’ll elaborate in successive articles, the dispute began in early June when Chinese People’s Liberation Army (PLA) engineers began constructing a road near the Indian border on a piece of territory disputed between China and Bhutan. India, perceiving this as an unacceptable change to the status quo with potentially serious strategic ramifications, crossed a settled and undisputed international border with its troops to block the PLA contingent from proceeding.
Also, India supports Bhutan's claim, so the they potentially sent troops as a show of support for them.
India has long supported Bhutan’s claim and, according to a release by the Indian Ministry of External Affairs on the ongoing standoff, reached an agreement in 2012 with China that existing “tri-junction boundary points” between the two countries and any third party would be “finalized in consultation with the concerned countries.”
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India and Bhutan have a special relationship, with New Delhi exercising considerable influence over the country’s foreign and defense policy historically and to this day; the two countries’ 1949 treaty of friendship was updated in 2007 by the two sides to give Thimphu additional autonomy in its foreign and security policy.
* All emphasis mine