What is language learning aptitude? How does it relate to learning a second language?

Aptitude testing is a system used by schools, the armed services, and some human relations departments to predict an individual’s level of success at learning a specific subject matter within a certain period of time. The military has for many years used the Armed Services Vocational Aptitude Battery as a way of directing recruits toward occupational specialties to which testing indicates they are most likely to find success. The Modern Language Aptitude Test (MLAT) serves the same purpose. Developed in the late 1950s, the MLAT became the standard system for determining the likelihood of individual students to learn within a reasonable period of time a foreign language. The test identifies strengths and weaknesses in each individual with respect to his or her innate skills at understanding the structure of languages. It is not intended to determine who is capable of learning a second language; rather, the MLAT enables analysts to identify those most likely to excel at language learning.
Whether or how well language aptitude testing works is a little subjective. Such testing is usually administered at the high school and college levels when students are already exposed to multiple languages and have developed a sense of individual skill regarding language learning. It has been determined, however, that the earlier children are taught foreign languages the more successful they will likely become at attaining fluency in those languages. Language aptitude testing in early childhood development is not always a good thing, though, because poor results can yield negative sentiments towards language learning. Telling children that they lack the skills necessary or desirable to learn a topic is obviously not conducive to those children’s future academic endeavors.
Irrespective of considerations regarding sensitivities to children who might be unfairly labeled insufficiently-skilled at language learning, there is no question that some people have more innate ability to learn foreign languages than others. This is no different than any other field of study or occupation. Some people are more naturally skilled at playing musical instruments or writing stories and essays then are other people. Language aptitude testing is simply a means of identifying those with the greater facility for learning languages, and it is successful. It is not, however, determinative of whether an individual is capable of learning another language.
http://lltf.net/aptitude-tests/what-is-language-aptitude/

https://www.military.com/join-armed-forces/asvab

https://www.teachingenglish.org.uk/article/language-aptitude