Tucking is when someone with a penis hides it to try and give the appearance of not having one. This is often done as a way to alliviate gender dysphoria for trans-feminine individuals.
Tucking can be done in various ways, such as with a gaff, tape, or tight/snug underwear. A gaff is a form of compression underwear used specifically for this purpose.

Potential risks may include; urinary trauma, infections, testicular compliants.

Some people have experienced involuntary erections while tucking due to the stimulation caused during the process. This is anecdotal, but something to take into consideration.

cantiqla - based in The United States
carmen liu lingerie - based in The United Kingdom
dys4ick - based in Australia
etsy - GenderBenderLLC - based in The United States
Etsy - StripItBack - based in Canada
gender gear - based in Canada
iguana trend - based in Poland
origami customs - based in the unceded traditional territory of the Kanien’kehá:ka (located in The United States)
rebirth garments - based in The United States
tcomfifits - based in Hong Kong
TomboyX - Collections - Tucking - based in The United States
ur basics - based in Canada
Your Open Closet - based in Canada

Guide to tucking
healthline - How Does Tucking Work and Is It Safe?
pointofpride - Gaffs 101
transcare - tucking handout
transhub - tucking
vice - The Girl's Guide to Tucking Your Dick
wikihow - How to Tuck and Tape
Male to Female: Bathing Suit
YouTube Video: Trick Tip: Making a Gaff!