Select Indent from the Block menu to insert a certain number of spaces at the start of each line in the selected portion of the current file. How many spaces are inserted depends on the "block indent" setting for the type of file you are editing. You can change this setting on the Editor page in the file type configuration. If the "block indent" setting is an integer multiple of the "tab size" setting, and the option "tab inserts spaces" is off, Block|Indent will use tab characters instead of spaces to indent the selection.
If the selection spans more than one line, you can also indent it by pressing Tab on the keyboard. The difference between using Block|Indent and the Tab key is that Block|Indent uses the "block indent" setting while Tab uses the "tab size" setting to determine the amount of indent. If "tab inserts spaces" is off, pressing tab will indent the selection by inserting exactly one tab character at the beginning of each paragraph.
Note that if word wrap is on, only the first line of each paragraph will be indented. Wrapped lines are only indented along with the first line if you've turned on Options|Indent Wrapped Lines.
If a rectangular block is selected, Block|Indent works differently. Instead of indenting the selected lines by inserting spaces or tabs at the start of the line, it will insert the spaces immediately before the first column in the selection, effectively shifting the block a number of spaces to the right.
Block|Move Lines and Columns