Raed665, assuming you're serious...
You don't really "read" rhythm-bars.
They have no real meaning - they're just placeholders, during which it's understood that the performer will improvise content within given or understood parameters (like chord symbols, or a specific style); from piece to piece they can be intepreted any number of ways.
A couple of things upon which one may depend:
1. If there are chord symbols above each measure of rhythm-bars - and there usually are - you'll know exactly what chord/tonality you're supposed to play, and for how long, based upon the measure and beat where the symbols change.
2. There will invariably be as many rhythm-bars in each measure as there are beats in that measure, i.e. in 4/4 metre there will be four rhythm-bars in each measure, in 3/4 there will be three, etc. Beyond that, they don't mean or even imply much of anything.
Otherwise you're left on your own to figure out what the rhythmic characteristics and other variables of what you're playing are supposed to be, based on the context.
Example -
You've got a guitar part for a piece, and the heading above the first bar reads SLOW GRUNGE. You know from this that the tempo of the piece is slow, and that the style is grunge; from that you can figure out the kind of groove you'll have to play. Above the measures of music - let's say the metre is 4/4 - are symbols that indicate the chord to be played in that measure: G (G major chord), Em (E minor chord), Bbmin7 (B-flat minor 7th chord), etc.
So if you see something that looks something like this:
You'll play, in a slow grunge groove:
a measure (4 beats) of G minor; followed immediately by...
a measure consisting of 2 beats of B-flat (major) and 2 beats of F (major); after which...
a measure (4 beats) of E-flat major-7; then...
a measure consisting of 2 beats of E7 and 2 beats of D minor-9; and finally...
a measure consisting of 3 beats of G2, and a 1 beat of G minor.
To do this effectively, you have to know not only how to find the indicated chords on the fingerboard, but also approximately what a "slow" tempo is, and what a grunge groove should sound like rhythmically and characteristically - including what volume/tone/amp/pedal setting combinations to use to create what would generally be recognised as a grungy sound on an electric guitar. Rather than write out what he wants the guitarist to play note-for-note, the composer/songwriter is confident that the guitarist will understand what is called-for and will improvise his part accordingly.
The same five measures of rhythm-bars would be intepreted very differently if the heading read BRIGHT SWING or MODERATE FUNK.
I'm sure someone else could have done a better job of this - and no-doubt more concisely - but there it is anyway. :D