Slash line is a colloquial term used to represent a player's batting average, on-base percentage and slugging percentage. Those three stats are often referenced together in baseball media with forward slashes separating them, which is where the term slash line comes from.
A slash line is presented with a player's batting average first, on-base percentage second and slugging percentage third (AVG/OBP/SLG). The latter two stats are added together to generate a player's OPS (on-base plus slugging).
"Slashed" is also sometimes used as a shorthand way to say a player produced a specific slash line, as in "Player A slashed .300/.400/.500 last season."