The 'line' is an arrow, meaning the field is a vector.
A vector has both size (or magnitude) and direction. As a field can point in different directions this information is also important.
The equation says that the electric field
The definition of electric field from my copy of
If a positive test charge
#q_o#is placed at point P and if an (electric) Force F acts on the stationary charge, an electric field E is present at P, where E is the vector satisfying the relation
The Equation of the pic you included, with your symbols, is
That equation is equivalent to the equation above from my college textbook.
#vecE#is the electric field. The units of #vecE#are Newtons/Coulomb.
#vecF_E#is the force that acts on a test charge due to the electric field it finds itself in. The unit of #vecF_E#is Newtons.
#q#is the charge on the test charge. The unit of q is Coulombs.
I hope this helps,