The 2S stands for two-spirit.

“Two-spirit” refers to a person who identifies as having both a masculine and a feminine spirit, and is used by some Indigenous people to describe their sexual, gender and/or spiritual identity. As an umbrella term it may encompass same-sex attraction and a wide variety of gender variance, including people who might be described in Western culture as gay, lesbian, bisexual, transsexual, transgender, gender queer, cross-dressers or who have multiple gender identities. Two-spirit can also include relationships that could be considered poly. The creation of the term “two-spirit” is attributed to Elder Myra Laramee, who proposed its use during the Third Annual Inter-tribal Native American, First Nations, Gay and Lesbian American Conference, held in Winnipeg in 1990. The term is a translation of the Anishinaabemowin term niizh manidoowag, two spirits.

Two-spirit people may also use terms from their Indigenous language to describe same-sex attraction or gender variance, such as winkt (Lakota) or nàdleehé (Dinéh). Some Indigenous languages do not have terms to describe sexual identities such as gay, lesbian, or bisexual. Many Indigenous languages are verb-focussed, and describe what people do rather than how they identify. For more information, please refer to the Canadian Encyclopedia.

https://lgbtqhealth.ca/community/two-spirit.php

And the + is typically added to the end of the acronym variations (e.g. LGBT+, LGBTIQ+, Etc.) to signify the inclusion of all other gender & sexual minorities as well.