fragmentation pattern
When you smash a clay pot, it does not shatter into random dust — it breaks along its weakest seams into a few characteristic pieces, and an expert could recognize the original pot from the shards. A molecular ion behaves the same way. Energized and unstable, it breaks at its weakest bonds into a predictable set of smaller charged fragments, and the collection of masses where those fragments land — the fragmentation pattern — is a fingerprint that reveals how the molecule was built.
Bonds break to give the most stable possible fragments, which is why the pattern is readable rather than random. A molecule preferentially loses a piece if doing so leaves behind a stable cation — a tertiary carbocation, an allylic or benzylic cation, or an oxygen-stabilized one — so chemists reason backward from a fragment mass to the group that must have left. Some losses are signatures: a gap of 15 from the molecular ion means a methyl group (CH3) broke off; a loss of 18 means water; a loss of 28 can mean carbon monoxide or ethene; a loss of 29 often means an aldehyde's CHO or an ethyl group. Each gap names a departing piece.
This is why mass spectra serve as a library for identification. Reference databases store the fragmentation patterns of hundreds of thousands of known compounds; a forensic or environmental lab matches an unknown's pattern against the library to name it. And for a chemist deducing a brand-new structure, the fragments are a guided tour through the molecule, each break confirming a particular bond and a particular group's location.
In the spectrum of 2,2-dimethylpropane and many branched alkanes, the base peak is often a tertiary carbocation; for tert-butyl compounds an m/z 57 fragment (the (CH3)3C+ cation) is prominent because that branched cation is especially stable.
Bonds break to give the most stable cation; the gap from M+ names the lost piece.
Only the charged fragment shows up in the spectrum; the neutral piece that breaks off is invisible. You infer the neutral loss from the gap between two peaks, not from a peak of its own.