A 'safe tile' in mahjong — a tile that can be discarded without much risk of letting another player win. Which tile is safe depends on the hand and the table situation.
この牌は安全牌だから、安心して捨てていい。
This tile is a safe tile, so you can discard it without worry.
終盤は安全牌を優先して切るのが基本戦術だ。
In the late game it's basic strategy to prioritize discarding safe tiles.
相手がリーチしているときは安全牌を探す。
When an opponent has declared riichi, look for safe tiles.
An idiomatic use meaning a 'no-risk choice' or a person who is easily handled/doesn't cause trouble — a safe, uncontroversial option or individual.
彼は会議でいつも安全牌を出す人だから、意見が頼りないと言われることもある。
He always plays it safe in meetings, so sometimes people say his opinions are unconvincing.
プロジェクトにリスクがあるときは、安全牌を選んで着実に進めよう。
When a project has risks, choose the safe option and proceed steadily.