A matrix used in a computation is singular or is very close to being singular (ill conditioned). Trying to solve a system of linear equations whose coefficient matrix is singular can lead to incorrect answers.
A program you called is using a matrix that is badly conditioned as one of the arguments to the backslash (\) or forward slash (/) operators, which are commonly used to solve systems of linear equations.
Stop MATLAB on the line where the warning occurs. Examine the line for instances of the two slash operators. If you find one, determine the condition number of the coefficient matrix (the matrix on the left side of the backslash (\) or on the right side of the forward slash (/) using the COND function. Large results for the condition number indicate the matrix is extremely ill-conditioned. You should verify (using the matrix multiplication operator (*)) that the result of solving the system is numerically reliable. For more information about condition numbers of matrices, see Chapter 2: Linear Equations of the Cleve Moler's Numerical Computing with MATLAB book, which discusses solving systems of linear equations.
Example demonstrating this error: