The void lookup limit was introduced in RFC 7208 and refers to DNS lookups which either return an empty response (NOERROR with no answers) or an NXDOMAIN response. This is a separate count from the 10 DNS lookup overall count.
As described at the end of Section 11.1, there may be cases where it is useful to limit the number of "terms" for which DNS queries return either a positive answer (RCODE 0) with an answer count of 0, or a "Name Error" (RCODE 3) answer. These are sometimes collectively referred to as "void lookups". SPF implementations SHOULD limit "void lookups" to two. An implementation MAY choose to make such a limit configurable. In this case, a default of two is RECOMMENDED. Exceeding the limit produces a "permerror" result.
This is meant to help prevent erroneous or malicious SPF records from contributing to a DNS-based denial of service attack.