You probably used a baseline before dtp programs were invented.
The baseline are a set of lines in the background on every page but are not printed. They are just like the printed sheet of paper that you slip behind a sheet of blank letter writing paper.
They have to set to be "set up" according to the body text you will be using. This might be a difficult task for someone new to indesign and then are just ignored and is one of the first things a professional will pick up. Setting the baseline means all lines text can be aligned with each other. This is really important when printing on semi transparent or thin paper.
Where the Baseline setup is located
When a new document in Indesign is opened, the baseline is set to 4,233mm ( 12pt or 1 pica) and this is ideal for the standard body text which is Times Roman with 10pt and a 12pt leading.
The baseline is set in the prefences dialog. Menu -> Edit -> Preferences -> Grid. You can also set the start value so it lines up with your magins.
Caesar would love Indesign
Default Times Roman sits perfectly on the default baseline. If you look in the text dialog box (strg + t) you can see the leading (12Pt).
Without a correctly setup baseline...
Everything begins to fall apart. The baseline has to be set for the standard body text you are going to use. As you can see here with 11pt Candara it does not at all work. The right hand column where the paragraph has be set to sit on the baseline results in double line spacing. Note in the text dialog for the leading is 13.2pt for 11pt Candara.
Setting the baseline to your leading
Change the value in the default settings to the value you require for the font type and size you will be using.
Perfect Alignment
All the lines of text now align perfectly. I also increase the gutters between the columns to same value as I used for the baseline. This ensures perfect squares are formed where the baseline and gutters cross. The baseline grid also helps alignment of other elements such as images and color bar with the text block without the need to set up raster grids for simpler documents.
Using multiples of the leading value
By increasing the values proportiontely - adding a intro (22pt) and title (88pt), and they also line up nicely on the baseline.
