These two look similar, with bars on a time axis, but they answer different questions. Picking the wrong one is the usual reason a plan looks unhelpful.

Gantt: the shape of a project

A row per item, bars spanning start to end, with dependencies between them. Use it for: is this plan realistic, and what slips if this slips?

A Gantt view in Atwork with client deliverables as bars from September to November, a line marking today, and a dependency from Brand guidelines refresh to Logo lockups for signage

Timeline: who is booked when

Rows are people or resources, and items sit on the row of whoever is doing them. Use it for: who is overloaded next week, and who is free?

Create one

Click + in the view switcher.
Choose Gantt or Timeline.
Gantt needs a date field. Timeline needs a date field and a Resource Field, the field that says who or what the work belongs to. A Select, Multi-Select, person or Relation field works.
Name it and save.

Start and end dates

Each bar comes from one date field that holds a range: its start date is where the bar begins, and its end date is where it stops. Change the field later under Settings → Date Field.

Dependencies (Gantt)

A dependency says one item cannot start until another is done.

Hover a bar until its link handle appears.
Drag from that bar to the bar that depends on it. An arrow joins them.
To remove a dependency, click the arrow.

Dependencies are stored in the sheet's Blocking and Blocked By fields, so the same link appears on both records. Creating a Gantt view turns on the sheet's dependencies, so both fields are there from the start, and they stay on while the sheet has a Gantt view. See Subtasks and dependencies.

Zoom

Gantt: Hour, Day, Week, Month, Quarter and Year.
Timeline: day, week, month and year.

Match the zoom to the question: weeks for a sprint, months or quarters for a roadmap.

Making them readable

Filter first. Everything you have ever done, on one axis, tells you nothing. Show one project, one quarter, or one team.
Color by status, so blocked or finished work is obvious at a glance.
On a Gantt, Progress by fills each bar from a number field, and Baseline by shows a second date range for the original plan, so drift is visible.

Notes

Items with no date cannot be plotted and will not appear. A near-empty timeline usually means missing dates, not missing work.
Dragging a bar changes the underlying dates on the record. Clicking it opens the record.
On a Timeline, Group by switches the Resource Field when the sheet has more than one field that could fill it.

Gantt and timeline views