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."
Jose Altuve produced a .346 average with a .410 on-base percentage and a .547 slugging percentage in 2017, winning the American League MVP Award. He can be said to have recorded a .346/.410/.547 slash line. Alternatively, Altuve can be said to have slashed .346/.410/.547 in 2017.