Marcato (from the Italian for ‘marked’) is an umbrella term that can apply to all instruments and therefore not a specific string instruction (like spiccato or sautille would be). In general terms, it is a stroke started with a fp (fortepiano) or sfz (sforzando) with a rest at the end of the stroke, meaning a note which starts with a bold attack, rapidly dying away with a gap before the next note. This can either be performed rapidly or slowly, but when performed slowly, there are big gaps between the notes. Marcato is a stroke where the bow does not leave the string in between notes and therefore is classified as a staccato bowing (as opposed to a spiccato bowing where the bow leaves the string in between notes).
My understanding is that the Martele stroke is the nearest to a true Marcato, in brief this stroke often performed in the upper half of the bow starts with a pressure (that brings the hair closer to the wood of the bow) and a rapid bow stroke with a simultaneous release of pressure. At the end of a stroke, there is a gap and the whole thing starts again!
A string arranger or composer would use marcato when they wanted to create a percussive sound with gaps in between. Slightly confusingly, an orchestrator could write dots underneath the notes to make them short – and this could be interpreted by a string player as lifting the bow off the string in between every note and could potentially be light in character. Marcato can also be written with a dot underneath the note, but on the other hand there is usually an accent with it and the length of the note is twice as long as the note is intended to be played (with the second half of the note being silence). To put it in context, Marcato is a heavier stroke and does not require the bow to leave the string. The analogy is that the spiccato can be like skipping or running and the marcato would be more like stamping or treading heavily, so a professional string player would usually know from the feel of the piece which stroke the composer intended.
Up until the mid 19th century, marcato was notated by a small downward v above the note, so if you come across very old sheet music that’s what it might be! It is less commonly used than spiccato.
As in all directions for bowings, context is everything and if written with a knowledge of what string instruments do, the players of a string section will always know exactly what stroke applies to a certain passage.