LGBT stands for Lesbian, Gay, Bisexual, and Transgender. The '+' indicates that there are more categories, and some even use LGBTQIA to include Queer, Intersex, and Asexual. Expanding this umbrella includes more people and brings attention to more marginalized people. Amplify their voices this month, and every month.
It is important to celebrate LGBT+ history because there is so much of it that goes unnoticed. Modern history is being made as we speak. We celebrate during this month because National Coming Out Day is October 11th in the USA. The UK celebrates this month during February.