One proposed definition of constraint is "A property of a trait that, although possibly adaptive in the environment in which it originally evolved, acts to place limits on the production of new phenotypic variants.
Any aspect of an organism that has not changed over a certain period of time could be considered to provide evidence for "constraint" of some sort.
Charles Darwin spoke of this concept in his 1859 book "On the Origin of Species", as being "Unity of Type" and went on to explain the phenomenon as existing because organisms do not start over from scratch, but have characteristics that are built upon already existing ones that were inherited from their ancestors; and these characteristics likely limit the amount of evolution seen in that new taxa due to these constraints.
The most common explanation for biological constraint is that stabilizing selection acts on an organism to prevent it changing, for example, so that it can continue to function in a tightly-defined niche.
For example, the acceleration caused by gravity places constraints on the minimum bone density and strength for an animal of a particular size.
As Rupert Riedl pointed out, this degree of functional constraint — or burden — generally varies according to position in the organism.