A detail view is a circle or rectangle drawn around a region on a parent drawing, with an arrow or label pointing to a separate enlargement of that region. The enlargement is drawn at a larger scale (commonly 2x or 5x the parent) so dimensions and small features are readable.
Detail views are common on gate drawings to show hinge mounts, drop bolt receivers, mitre cut details, and footplate bolt patterns. The parent view shows the gate context. The detail view shows the geometry the welder needs to actually make the part.
Label convention: each detail gets a unique reference (Detail A, Detail B, etc) cross-referenced between the parent and the detail view. The detail view's title block usually carries its own scale, distinct from the sheet scale.