It means that your heart capacity is, at the moment of testing, below what is considered normal (which is 100%) for your age and gender.
This does not mean that you have a heart problem though. There is a lot of variation from person to person.
A lot more is measured during the stress test.
Your cardiologist will give you his view, based on the whole set of measurements.
Not sure but can say of the many echocardiograms I have had, and keeping an eye on the size of the atrium (left in particular) chamber, we get different sizes each time.. not a big difference, but it emphasizes that the test is not anywhere near 90 % accurate. It this applies to your tes I can conclude you may be worse, but let's hope better, say really 90%.
The doctor must not have considered the result threatening, (assumption) ... so if you feel okay, just go with it. Okay means you can walk, you're not sitting around all the time... exercise may also be a way to increase the capacity.. just my opinion.