The bond energy also known as bond dissociation energy is the energy required to break a covalent bond.  The bond energy is a measure of the strength of a bond and is the enthalpy change for a gas  phase reaction in which the bond breaks.  These energies are usually expressed in kilojoules per mole of bonds and are compiled in tables like the one below.  

The bond energy between a given pair of atoms varies slightly from one compound to another.  For example the bond dissociation energy for the C-H bond in methane, CH₄ is 435 kJ while in ethane, C₂H₆ it is 410 kJ.  Because these variations in strength are not very great, the average bond energy is a good guide to the strength of a bond in any molecule. It is these average bond energies that are given in tables like the one below. 

Bond Energies

C=C	602	C=N	615	C=O	745(799 in CO2
CC	835	CN	887	CO	1072
N=N	418	N=O	607	S=O (in SO2)	532
NN	942	O2	494	S=O (in SO3)	469

When possible it is preferable to use thermochemical data calculate the enthalpy of a reaction.  If the thermochemical data are not known, however, bond energies can be used to give you an estimate of the heat of reaction.  To illustrate this consider the following example.

Use bond energies (Table above) to estimate H (kJ) for the following gas-phase reaction:
N₂ + 2 F₂ ==> NF₂-NF₂
Solution
In the reaction, a NN triple bond and two F-F bonds are broken. One N-N single bond and four N-F bonds are formed.
H = [BE(NN) + 2BE(F-F)] - [BE(N-N) + 4BE(N-F)]
H = [942 + 2(155)] kJ - [167 + 4(283)] kJ = -47 kJ