Sow

Sow[e] specifies that e should be collected by the nearest enclosing Reap.

Sow[e, tag] specifies that e should be collected by the nearest enclosing Reap whose pattern matches tag.

Sow[e, {tag1, tag2, ...}] specifies that e should be collected once for each pattern that matches a tagi.

Examples

Collect values with Reap and Sow:

Reap[Do[Sow[i^2], {i, 5}]]
(* {Null, {{1, 4, 9, 16, 25}}} *)

Use tags to separate values:

Reap[Do[Sow[i, If[EvenQ[i], "even", "odd"]], {i, 6}], {"even", "odd"}]
(* {Null, {{{2, 4, 6}}, {{1, 3, 5}}}} *)

Please visit the official Wolfram Language Reference for more details.