Complex analysis and large repertoire trees often have very deeply nested variations. To make the notation easier to read ChessBase provides a folding facility. This is activated with a right click in the notation.
In the fold menu you can select the following:
Fold all: The notation only displays short lines. At points where sublines are present there is this symbol: 
. Click on this to unfold individual sublines.
Unfold all: The variations are all unfolded. At the branching points you will find this symbol . Click on this to fold away individual variations.
Disable folding: This switches off folding completely. All folding buttons disappear from the notation.
Some things to note:
When you replay a "folded" game the variations are still offered in the variation window and are automatically unfolded if you enter them.
Pressing Delete folds the current variation complex and jumps to the last branching point, which appears in the variation menu.
Pressing "+" and "–" fold and unfold the next branch (unless an analysis engine is running).