A marker is an ink-filled pen with a wide tip. Kids often use washable markers, while adults are usually trusted with the permanent type.
Permanent markers contain ink that can't be washed away, and they're useful for labeling and marking things like cardboard boxes and file folders. Another kind of marker is a highlighter, which you can use to make important words or phrases in a book stand out with a line of translucent neon ink. A different meaning of marker is "something that marks a route or specific place." Markers along a hiking trail help keep you from getting lost.