You need to know: Polynomial in n variables, degree of a polynomial, complex numbers.
Background: Decomposable form is a polynomial where the coefficients
are non-zero complex numbers.
The Theorem: On 24th January 2000 Jeffrey Thunder submitted to Annals of Mathematics a paper in which he proved, among other results, that for every decomposable form F of degree d in n variables with integer coefficients, the number of integer solutions to the inequality
is either infinite or at most
, where
is an effectively computable constant depending only on n and d.
Short context: Counting integer solutions to equations and inequalities is an old theme in mathematics. Case d=2 of the Theorem was proved by Mahler in 1933, but progress in estimating for d>2 was limited. The Theorem estimates
for all d, confirming 1989 conjecture of Schmidt.
Links: Free arxiv version of the original paper is here, journal version is here. See also Section 1.3 of this book for an accessible description of the Theorem.