Context: In both commercial and open-source software development, newcomers often join the development process in the advanced stages of the software development lifecycle. Newcomers frequently face barriers impeding their ability to make early contributions, often caused by a lack of understanding. For this purpose, we have developed an LLM-based tool called SPAC-B that facilitates project-specific question-answering to aid newcomers' understanding of software projects. Objective: Investigate the LLM-based tool's ability to assist newcomers in understanding software projects by measuring its accuracy and conducting an experiment. Method: In this study, a case study is conducted to investigate the accuracy of the tool, measured in relevance, completeness, and correctness. Furthermore, an experiment is performed among software developers to test the tool's ability to help newcomers formulate better plans for open-source issues. Results: SPAC-B achieved an accuracy of 4.60 in relevance, 4.30 in completeness, and 4.28 in correctness on a scale from 1 to 5. It improved the combined mean score of the plans of the 10 participants in our experiments from 1.90 to 2.70, and 8 out of 10 participants found the tool helpful. Conclusions: SPAC-B has demonstrated high accuracy and helpfulness, but further research is needed to confirm if these results can be generalized to a larger population and other contexts of use.