LGBTQ is an abbreviation that originated in the 1990s; it was created to be more inclusive of diverse groups. LGBTQ stands for lesbian, gay, bisexual, transgender, and queer (and/or questioning) individuals/identities. LGBTQ literature refers to works produced by or for the LGBT community which involves characters, plot lines, and/or themes portraying LGBTQ issues and identities.
There has been some controversy over works with these themes written by cis-gendered and/or heterosexual individuals, in other words, people who themselves are not LGBTQ. You may want to keep that in mind as you choose books you would like to read.