What do we learn from an isotope peak (or M+2 peak) in a mass spectrum?
Each element has different natural abundances for its isotopes. Two peaks with approximately 1:1 ratio indicates the presence of bromine. $\ce{^{79}Br}$ is the most common isotope, corresponding to $0.5069$ of naturally occurring bromine.  The isotopic composition for $\ce{^{81}Br}$ is $0.4931$. 

What this means is that, for a molecule contaning a single bromine atom, approximately, half of the molecular ions will have $\ce{^{79}Br}$ and the other half will have $\ce{^{81}Br}$, resulting in two peaks with roughly the same intensity with $m/Z$ differing by two units. 

References:

[1] https://www.nist.gov/pml/atomic-weights-and-isotopic-compositions-relative-atomic-masses

[2]https://chem.libretexts.org/Bookshelves/Analytical_Chemistry/Supplemental_Modules_(Analytical_Chemistry)/Instrumental_Analysis/Mass_Spectrometry/Organic_Compounds_Containing_Halogen_Atoms