In Counter-Strike, a ninja plant occurs when a player on the T side plants the bomb very early into the round, onto a defended bomb site, unopposed.
Ninja plants are commonly achieved using a sneaky smoke bomb, with a clever choice of movement, or simply by pure luck. Ninja plants are usually embarrassing for the CTs, as it shows their lack of attention and/or skill.
Seeing the poor positioning of opponents, the T player took a deep breath before sneaking onto bomb site A, managing to ninja plant the bomb before the CTs could get him on his crosshair. He then quickly hid behind a box, waiting for his teammates to arrive at the bomb site.