There is a dispute regarding the performance of mitzvoth in general, as to whether they require any kavanah at all or not. Halachipedia outlines 2 types of Kavanah:
the concentration on the mitzvah to the exclusion of anything else and
a sincerity of heart about what one is saying
prior to performing a mitzvah one should think that I am hereby going
to fulfill a particular Mitzvah
The site mentions rules regarding what happens if you recited Biblical (De'oraita) mitzvoth (which Shema is) without the proper kavanah. Shema, however, has some interesting exceptions to the standard rules.
Some say that by Mitzvot that involve speech like Kriyat Shema one
needs Kavana even according to the Rishonim who hold Mitzvot don’t
need Kavana. However most Achronim hold that there’s no
differentiation. [13] Nonetheless, we pasken like the Rishonim who
hold Mitzvot need Kavana. [14]
Besides Kavana that one has to fulfill the positive Mitzvah to say
Shema, one needs to understand what one is saying in the first pasuk
of Shema because of the Kabalat Ol Malchut Shamayim (acceptance of
yoke of heaven) and Yichud Hashem (knowing the Hashem is one). [19]
However, in the rest of Shema one only needs Kavana to say the words
(not to be Metasek, accidentally doing the action). [20]
I've left the footnote numbers in the quote so that you can more easily cross-ref them when you go the site.