TGE is the umbrella term for launches that were once called an ICO, an IEO on an exchange, or an IDO on a DEX. It marks the start of trading and the beginning of the token-unlock clock.
Launch day pricing is unreliable. Float is at its smallest, liquidity is thin, and early recipients with a near-zero cost basis are deciding whether to sell into whatever demand appears.
Do the arithmetic before participating. A sale at $0.10 with 50m of 1bn tokens circulating is a $5m raise implying a $100m fully-diluted-valuation. Also check the jurisdiction: public token sales are restricted or illegal in many places, and participation may carry legal risk.
Related: presale, fully-diluted-valuation, token-unlock, exchange-listing-effect