Reveal lines on click

A Beamer frame can be more than one physical slide. When you reveal bullet points one at a time, Beamer generates several PDF pages from the same frame, each showing a bit more, and advancing in the PDF viewer steps through them. These generated pages are called overlays, and controlling them is the main skill that separates a static deck from a paced talk.

\begin{itemize}
 \item<1-> Always visible from slide 1
 \item<2-> Appears on slide 2
 \item<3-> Appears on slide 3
\end{itemize}

\pause % simpler step-through

Overlay specifications

The angle-bracket notation after \item is an overlay specification, and it states on which overlays of the frame the item is visible. The number is the overlay index within this frame, and the hyphen makes it a range: <2-> means from overlay 2 onward, so the item appears on the second step and stays. A bare <2> would mean overlay 2 only, so the item would appear and then vanish. You can also give lists like <1,3>. Beamer creates as many overlays as the highest number mentioned, so the frame above produces three PDF pages.

Overlay specifications work on more than items. \textbf<2->{key term} bolds text from overlay 2 onward, and \only<2>{...} includes its content solely on overlay 2. Because \only removes the content entirely on other overlays, the surrounding layout reflows, which makes it the tool for replacing one figure with another in the same spot: give two \only blocks the same position and consecutive specifications. Its cousin \uncover reserves the space while hiding the content, so nothing shifts.

The simpler tool

\pause splits the frame at that point: everything before it shows immediately, everything after waits for the next step, and repeated \pause commands create successive steps. It needs no numbers and no planning, which makes it the right default for a plain top-to-bottom reveal. Reach for explicit specifications only when the reveal order differs from the source order, or when items must appear and disappear.

A useful shortcut when every item in a list should step in order is \begin{itemize}[<+->], which assigns increasing overlay numbers automatically.

Restraint

Overlays are easy to overuse. A deck where every bullet on every slide clicks in one at a time exhausts both the presenter, who must click constantly, and the audience, who cannot read ahead. Reserve stepped reveals for the few places where pacing carries an argument, such as building up a derivation or contrasting before and after. Keep in mind that each overlay is a real PDF page, so handout copies balloon unless you compile with \documentclass[handout]{beamer}, which collapses every frame to its final overlay. Frame basics are covered in a first Beamer deck.