Tactical clothing is a feature in Escape from Tarkov that allows you to customize the appearance of your PMC character to obtain reduced visiblity or just look more fancy.
It can be accessed via Ragman's "services" tab at the top of the trading menu.
Clothing is separated by upper and lower pieces. They can be worn in any combination, although BEAR and USEC operators each have their own parts. Besides unlocking them through character progression, some can also be permanently unlocked through the purchase of expansions on the Escape from Tarkov website.
In the tactical clothing menu, right clicking a clothing option will show the character model with it, while right clicking on the character model toggles the currently equipped gear to be hidden or shown.