数学看不懂串

A vertical vector on a fibre bundle ε with base M is a tangent vector on ε which is tangential to the fibres: that is,X(π*f)=0 for any f∈C^{ \inf }(M),where π*f=f \circ π.
We denote the bundle of vertical tangent vectors by Vε;it is a subbundle of the tangent bundle Tε.

这是在讲一件什么事情啊？从句太多导致看不懂意义了。数学书好讨厌......（趴）