It's not about having a set income from employment, it is about whether she has a certain amount left over from all sources of income once certain deductions have been made, so that she will not need to have additional recourse to public funds as a result of her husband joining her in the UK. The amount she needs to have left is based on the income support rates for a family of her size.
Specifically, she needs to show that she has left, after rent/mortgage, council tax and loan repayments have been deducted:
105.95 per week for a couple
62.33 per week for each child
= £292.94 per week or £1269.41 per calendar month for the family after deductions as explained above.
She can include any income she receives from benefits such as tax credits, child benefit etc.
The above rates are assuming she is not eligible for higher rate benefits for example due to a disability.
See also
spouse visa queries
