It means to move righthandwards, in the direction of the sun, clockwise. It now mainly conjures up associations with witchcraft, as it’s the much rarer converse of widdershins.
Trying to define it unambiguously immediately runs into the fundamental problem of how to explain the difference between left and right (clockwise is fine, unless you only know about digital clocks, or you’re one of those jokers who has one that runs backwards; sunwise works in the northern hemisphere only; just try explaining to an alien visitor which is right and which left, using words only).
In immediate origin deasil is a Gaelic word that derives from the same root as the Latin dexter, “right, right-handed” which even then could mean “skilful” (hence our word dextrous). In witchcraft, including modern Wicca, to move deasil is to invoke positive qualities.
As an example of its associations, here is Sir Walter Scott, in Chronicles of the Canongate: “In the meantime, she traced around him, with wavering steps, the propitiation, which some have thought has been derived from the Druidical mythology. It consists, as is well known, in the person who makes the DEASIL walking three times round the person who is the object of the ceremony, taking care to move according to the course of the sun”.