Well... that was vague and woolly
Open source has a wide range of meanings, including "You can read the source code, but if you do and later make money from software Microsoft can sue you for copyright and patent infringement" (I am not kidding, Microsoft really did make software available under license like that).
Ignore what people call it at look at the license. The big two are:
BSD: do what you want but mention us so we can demonstrate that we did something useful with our last funding and get another grant next year.
GPL: do what you want, but if you distribute GPL software you must make the source code available to the recipients under the GPL license.
The massive advantage to absolutely everyone is licenses like these prevent lock-in:
Use our cheap backup software. You want to restore from backup encoded in our secret format? £££ (British gas fell for that one)
Our software is cheap. Want it to do a little extra? £. and a bit more? ££ and something else: £££ ...
Once you are locked in, the price of the most trivial change is just less than the cost of replacing the whole thing.
If you have the source code _and_ the right to create derivative works, the cost of any change can be the lowest competent bidder.