This command defines a tree constraint. The format for the constraint command is
constraint <constraint name> <probability> = <list of taxa>
A list of taxa can be specified using a taxset, taxon names, or taxon numbers. A probability must also be specified. For now, MrBayes ignores this probability value and treats the constraint as an absolute requirement of trees. That is, trees that are not compatible with the constraint have zero prior (and hence zero posterior) probability.
Future releases of MrBayes will use the probability value to determine how much more probable a tree is that contains the constraint than a tree without the constraint. For example, the following command
constraint example 100 = taxon_2 taxon_3
defines a constraint called "example" that includes two taxa. In future releases of MrBayes, trees that contain a clade with those two taxa together will have a prior probability that is 100 times that of trees without the constraint. In the current version, the probability value will be ignored and trees without the two taxa together will not be sampled.
If you are interested in inferring ancestral states for a particular node, you need to constrain that node first using the 'constraint' command. For more information on how to infer ancestral states, see the help for the 'report' command.
It is important to note that simply defining a constraint using this command is not sufficient for the program to actually implement the constraint in an analysis. You must also specify the constraints using 'prset topologypr = constraints (<name of constraint>)'. For more information, see the help on the 'prset' command.