What exactly is an "academic plan"?

I suspect that even if the TERM is not defined, that knowing the JOB will help you make a proper submission. Is it a postdoc or lecutureship or tenure track? Even if the job description is scanty (but why would you invest time in a detailed submission for a job that you don't know what it is), you can make some guesses based on the school, group, etc. Is the school a teaching or research university?

So, I really don't understand coming at this thing with a "can you explain this term to me" versus giving more context, clues. This is both you might answer the question yourself or how we might help you.

Also, I would reach out say you are interested and ask for guidance.

I was hoping somebody with more experience would answer this, since I am also currently in the process of applying for my first Lecturer position. You say you are applying for a positions in Spain, so take care that my experience (and the guidance I received) was targeted at applications in UK universities.

I would assume your academic plan is what UK universities refer to as (X-year) research plan (typically, they would ask for a 3-year or a 5-year research plan with the application). They broadly want to know:

• that you are able to plan ahead for 3-5 years and plan your own work (now that you are moving away from a "supervised" postdoc position to a Lecturer position)
• broad ideas or topics you plan to tackle in those 3-5 years. No specific details, just 2 or 3 broad research ideas, where you show that you plan to tackle relevant and interesting problems (well, that's what you have to convince the interview panel of)
• how you plan to fund your research activities, get students and other resources. At least for UK applications, each of the 2-3 research topics presented should be bundled up with a plan to get it funded. Again, while you don't oblige yourself to apply to specific grants they want to see that you are familiar with the system of obtaining funding and know how you are going finance your support research staff.
• all of the above specifically in the context of the position, team, department you are applying for (so, it should be re-done for every application you are doing, highlighting the points pertinent to the specific research group you are applying for).

Edit: (I understand that Spanish is spoken outside of Spain as well, so this might not apply fully, but, according to my Spanish colleague):

• he said he would interpret "propuesta de plan academico" as not only a research plan, but rather a document that would also explain how your research activities will benefit your teaching performance and student engagement, and vice versa (i.e. how your research can benefit from your teaching activities).