Chest binding is a process of flattening one's chest. People experience gender dysphoria and chest binding helps them align with the gender they resonate with. It is a form of gender expression.
Some of the ways to bind one's chest is by shirt layering, commercial binders, sports bra and kinetic tapes. Using material which are not skin friendly like duct tapes can cause serious problems and some people are also tempted to use a tight strap of cloth which can lead to breathing problems and blood clots if tied too tightly.