You can use the filter property to retrieve a subset of values from CRM. In this example only the accounts where the company name equals CompanyName are returned.
$.ajax({
url: Xrm.Page.context.getClientUrl() + '/api/data/v8.0/accounts?$filter=name eq CompanyName',
headers: {
'Accept': 'Application/json'
}
}).done(function (result) {
var accounts = result.value;
});