An obfuscatory leftist phrase. And therefore used by Obama the Mendacious. Why obfuscatory? Because it elides an important distinction between those terrorists who are truly homegrown such as Timothy McVeigh and those who, while born in the USA, such as Omar Mateen, derive their 'inspiration' from foreign sources. Mateen's terrorism comes from his understanding of what Islam requires, namely, the liquidation of homosexuals. There is nothing homegrown about Islam. This in stark contrast to the American sources of McVeigh's terrorism.
It is perfectly obvious why liberals and leftists use 'homegrown terrorist' in application to the likes of Mateen: they want to deflect attention from the real problem, which is radical Islam.