ABL has different licenses for different versions of its products:
The standard edition of the ABL engine is licensed using the ABL Public License, which is essentially equivalent to the Mozilla Public License 1.1.
The professional edition is licensed using our Commercial License, unless some other agreement has been reached.
Generally speaking, the standard edition is open source, and you can use it in your project without asking anyone's permission. You can also include it and redistribute it to your heart's content, with few restrictions.
You should read the complete license to get all the details, but overall, the only serious restrictions are that any changes you make to the code cannot be proprietary. We welcome contributions, bug fixes, enhancements, and so on, but we don't want these contributions to be restricted or sold. The basic ABL engine is free to all, and we want it to remain that way.
The commercial license is somewhat more restrictive. For obvious reasons, you can't redistribute the product, and you do have to pay for it (except for small projects), but you do get access to the source, and you can contribute to it.
Once again, you should read the complete license for all the gory details, but in general, we want to be as open and fair as we possibly can. If you think we didn't get it right, please let us know. We're always interested in your feedback and ideas.