This command joins each of its arguments together with spaces after trimming leading and trailing white-space from each of them. If all of the arguments are lists, this has the same effect as concatenating them into a single list. Arguments that are empty (after trimming) are ignored entirely. It permits any number of arguments; if no args are supplied, the result is an empty string.
% concat a b {c d e} {f {g h}}
a b c d e f {g h}
it will also concatenate things that are not lists, as can be seen from this session:
% concat " a b {c " d " e} f"
a b {c d e} f
Note also that the concatenation does not remove spaces from the middle of values, as can be seen here:
% concat "a b c" { d e f }
a b c d e f
(i.e., there are three spaces between each of the a, the b and the c).