The word queer can mean different things to different people, but the most common definition is that it is someone who is not cishet, or someone who has different experiences of orientation, gender and/or sex.
One of the first flags to be explicitly called a “queer flag” was designed by Pastelmemer in 2015. There are not many alternative flags, but this one is still the most popular.
On this flag:
The shades of pink next to each other and the shades of blue next to each other symbolise same-sex attraction.
The orange and green colours represent non-binary people.
Black and white represent asexual, aromantic and agender persons.