diff --git a/modules_and_packages.ipynb b/modules_and_packages.ipynb
index 301cdab..f341d20 100644
--- a/modules_and_packages.ipynb
+++ b/modules_and_packages.ipynb
@@ -731,6 +731,14 @@
     "\n",
     "To make the package installable, we need to do a few more things.\n",
     "Typically, this is done by creating the `pyproject.toml` file.\n",
+    "`pyproject.toml` is a configuration file that contains the build system requirements and the package metadata.\n",
+    "It is used by the build tools like `flit` or `poetry` to build and install the package.\n",
+    "It is also used by other tools like `black` and `mypy` to configure their behaviour.\n",
+    "\n",
+    "The [`toml` format](https://toml.io/) is a configuration file format that is easy to read and write for humans (unlike `json`).\n",
+    "It is quite flexible but not as complex as the `yaml` format.\n",
+    "Another advangtage of the `toml` format is that it is pre-installed in Python, so you don't need to install any additional packages to use it.\n",
+    "\n",
     "\n",
     "Here is a minimal example of the `pyproject.toml`: \n",
     "\n",
@@ -814,7 +822,6 @@
    ]
   },
   {
-   "attachments": {},
    "cell_type": "markdown",
    "metadata": {},
    "source": [
@@ -839,7 +846,6 @@
    ]
   },
   {
-   "attachments": {},
    "cell_type": "markdown",
    "metadata": {
     "tags": []