Hi. I want to interpret the coefficient on the educational categories, where the referent category is primary education and wage is measured in pounds. Kindly help.

Code:
. heckman wage c.age i.gender i.education, select (i.married c.age) twostep
 
Heckman selection model -- two-step estimates   Number of obs     =     26,879
(regression model with sample selection)              Selected    =     17,820
                                                      Nonselected =     11,924
 
                                                Wald chi2(10)     =     715.34
                                                Prob > chi2       =     0.0000
 
------------------------------------------------------------------------------------------
                 wage   |      Coef.   Std. Err.      z    P>|z|     [95% Conf. Interval]
-------------------------+----------------------------------------------------------------
wage                     |
                  age    |  .2118695    .0220511     9.61   0.000     .1686502   .2550888
                         |
                   gender|
               2 male    |  .4308575    .074208      5.81   0.000     .2854125   .5763025
                         |
               education |
        2 middle school  |  .4473249    .0287417     15.56  0.000     .3909922   .5036576
          3 high school  |  .0583645    .0109742     5.32   0.000     .0368555   .0798735
           4 university  |  1.185526    .0931944     12.72  0.000     1.002868   1.368184
                         |
                   _cons |  -2.467365   .1925635    -12.81  0.000     -2.844782  -2.089948
-------------------------+----------------------------------------------------------------
select                   |
                 married |
               1.married |  .9825259    .0538821   18.23   0.000     .8769189    1.088133
                  Age    |  .1426833     .0018027   79.15   0.000     .13915     .1462165
                         |
                   _cons |  -2.680182   .0358616   -74.74   0.000    -2.750469   -2.609894
-------------------------+----------------------------------------------------------------
/mills                   |
                  lambda |  4.001615    .6065388    6.60    0.000     2.812821    5.19041
-------------------------+----------------------------------------------------------------
                     rho |   -0.37655
                   sigma |  7.2459831
------------------------------------------------------------------------------------------